1. Field of the Invention:
This invention relates to an electronic tuning circuit for an AM radio receiver including a radio-frequency tuning circuit and a local oscillator circuit wherein a variable capacitance diode is used as a tuning element, whereby tracking error is reduced.
2. Description of the Prior Art:
In order to have a better understanding of the present invention, description will first be made with reference to FIG. 1, wherein there is shown a conventional electronic tuning circuit for an AM receiver, which includes a radio-frequency tuning circuit 10 and a local oscillator circuit 20. A tuning voltage V.sub.T which is derived from a variable voltage source 40 on the basis of a PLL synthesizer system or the like, is applied to the radio-frequency tuning circuit 10 and local oscillator circuit 20 through resistors R.sub.10 and R.sub.11 respectively so that tuning to a desired frequency is achieved. The radio-frequency tuning circuit 10 comprises a tuning coil L.sub.10 ; a trimmer capacitor C.sub.10 and capacitor C.sub.S10 which are connected in parallel with the tuning coil L.sub.10 ; a capacitor CP.sub.10 connected at one end to a connection point between the tuning coil L.sub.10 ; the trimmer capacitor C.sub.10 and the capacitor C.sub.S10 ; and a variable capacitance diode D.sub.10 connected to the opposite end of the capacitor C.sub.10l . The local oscillator circuit 20 comprises an oscillation coil L.sub.11 ; a padding capacitor C.sub.P11 connected to one end of the oscillation coil L.sub.11 ; and a trimmer capacitor C.sub.11, capacitor C.sub.S11 and variable capacitor diode D.sub.11 which are connected in parallel with each other between the other ends of the padding capacitor C.sub.P11 and oscillation coil L.sub.11.
With an AM receiver, it has been the usual practice that tracking adjustment is carried out by adjusting the capacitance of the trimmer capacitors C10, C11 and the inductance of the coils L10, L11 of the electronic tuning circuit after the components of the electronic tuning circuit have been individually mounted onto a printed circuit board, together with other units such as amplifiers and so forth. However, such a procedure is disadvantageous in that tracking errors tend to occur inevitably from a theoretical point of view. More specifically, with a superheterodyne receiver, its local oscillator circuit is arranged to produce oscillation at a frequency which is higher than the receiving frequency by an amount equal to the intermediate frequency (450 KHz). Thus, to receive a signal of a frequency in the medium wave (MW) band (522 KHz to 2079 KHz), it is required that the frequency of oscillation produced at the local oscillator circuit be varied in the range from 972 KHz to 1629 KHz. A comparison of the frequency variation ratios indicates that the frequency variation ratio A.sub.1 of the radio-frequency tuning circuit 10 is given as follows: ##EQU1## and that the frequency variation ratio A.sub.2 of the local oscillator circuit 20 is given as follows: ##EQU2##
From the equations (1) and (2), it will be seen that the frequency variation ratio of the local oscillator circuit 20 is different from and lower than that of the radio-frequency tuning circuit 10. Usually, the variable capacitance diodes employed with the radio-frequency tuning circuit 10 and local oscillator circuit 20 are of an equal capacitance. Thus, difficulties have been experienced in an attempt to reduce tracking errors due to the fact that the frequency variation ratios of the radio tuning circuit 10 and local oscillator circuit 20 do not conform to each other for the reason mentioned above.
Referring now to FIG. 7, curve (a) shows tracking error which tends to occur when the padding capacitor C.sub.P11 is removed from the local oscillator circuit 20 of FIG. 1; curve (b) indicates tracking error which tends to occur when the local oscillator circuit 20 comprises the oscillation coil L.sub.11, variable capacitance diode D.sub.11 and padding capacitor C.sub.P11 ; and curve (c), which corresponds to a combination of the curves (a) and (b), represents tracking error which tends to occur with the conventional electronic tuning circuit.
With the conventional AM receiver, it has been usual practice to perform tracking adjustment after electronic components have been mounted onto a printed circuit board. Thus, at the final adjustment step, it may happen that tracking error goes beyond the control limits. If that happens, then all the components mounted on the printed circuit board must be discarded, or alternatively, the defective component or components must be replaced, and tracking adjustment must again be performed. Obviously, such a conventional arrangement is disadvantageous in that too many components are used for performing tracking adjustment so that too many adjustment steps are involved, which leads to an increase in the manufacturing cost.
Furthermore, with the conventional arrangement of AM receiver, even if tracking adjustment has been performed, tracking error occurs theoretically in the range of 3 KHz to 10 KHz; thus, such a conventional arrangement is not applicable to an AM stereo receiver in which it is required that tracking error be reduced over a wide frequency range. The use of the conventional electronic tuning circuit with AM stereo receiver is disadvantageous in that it results in poor stereo separation so that distortion is caused.